The overall objectives of the present study were directed towards elucidation of the origin of human pandemic influenza viruses. Further evidence on the origin of Hong Kong influenza virus from "animal" sources was provided by showing that the hemagglutinin molecules of the Hong Kong, equine 2 and duck/Ukraine strains of influenza virus do not possess N-terminal aspartic acid on the HA1 polypeptide chain. All other influenza viruses examined possessed N-terminal aspartic acid on the HA1 polypeptide chains. Influenza viruses from feral avian species replicate in the cells lining the intestinal tract and are spread in water supplies. The RNAs of the Hswl N1 influenza virus from man and from swine are heterogeneous. The RNAs of a series of influenza virus recombinants (Hav5 Nav6 x ON1) were examined and segments 3 and 5 from the Hav5 Nav6 virus were always associated with virulence. The matrix protein of influenza viruses was detected on the surface of virus infected cells.